The Generations of Mexicans that moved to the US perhaps doesn't see the move as you and most Americans do. It appears its more like rebalancing, since its perceived the land was stolen.
Discussion
When the current SW USA was part of Mexico, it was occupied by the native tribes of the area and the Spanish missionaries. So, anyone who arrived to the area from south of the current border would not be able to make a realistic claim that the land is "theirs".